Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly, to an implantable dual-chamber pacemaker configured to provide PVC-protected hysteresis and automatic AV interval adjustment in the DDI pacing mode.
The basic function of the heart is to pump blood (circulate) throughout the body delivering oxygen and nutrients to the various tissues while removing waste products and carbon dioxide. The heart is divided into four chambers comprised of two atria and two ventricles. The atria are the collecting chambers holding the blood which returns to the heart until the ventricles are ready to receive this blood. The ventricles are the primary pumping chambers. The pumping function of the heart is achieved by a coordinated contraction of the muscular walls of the atria and the ventricles.
The heart is commonly thought of as having two sides, the right side and the left side. Blood returning to the heart from the body (legs, arms, head, abdomen) returns to the right atrium. From there, it goes to the right ventricle from which it is pumped to the lungs. In the lungs, the carbon dioxide collected from the body is exchanged for oxygen.
The oxygenated blood then travels to the left atrium from which is enters the left ventricle. The left ventricle is the major pumping chamber circulating the blood to the remainder of the body.
The atria are more than simple collecting chambers. The atria contain the heart's own (natural, native or intrinsic) pacemaker that controls the rate at which the heart beats or contracts. In addition, the atrial contraction helps to fill the ventricle, further contributing to optimal filling and thus maximizing the amount of blood which the heart is able to pump with each contraction. Hence, atrial contraction is followed after a short period of time (normally 120 to 200 ms) by ventricular contraction.
The period of cardiac contraction during which the heart actively ejects the blood into the arterial blood vessels is called systole. The period of cardiac relaxation during which the chambers are being filled with blood is called diastole. Atrial and ventricular systole are sequenced allowing the atrial contraction to help optimally fill the ventricle. This is termed AV synchrony.
A cardiac cycle comprises one sequence of systole and diastole. It can be detected by the physician by counting the patient's pulse rate. It is also reflected by the cardiac rhythm as recorded by an electrocardiogram. The electrocardiogram (ECG) records the electrical activity of the heart as seen on the surface of the body. The electrical activity refers to the cardiac depolarization in either the atrium and/or ventricle. On the ECG, the atrial depolarization is represented by the P-wave while the ventricular depolarization is represented by the QRS complex, sometimes abbreviated as an "R-wave". The electrical depolarization triggers or initiated the active muscular contraction. Once the cardiac cells are depolarized, they must repolarize in order for the next depolarization and contraction to occur. Ventricular repolarization is represented by the T-wave. Atrial repolarization is rarely seen on an ECG as it occurs at virtually the same time as the R-wave and is hidden by this large electrical signal.
A normal heart rate varies between 60 to 100 beats per minute (bpm) with an average of 72 bpm resulting in approximately 100,000 heart beats per day. The heart beat normally increases during period of stress (physical or emotional) and slows during periods of rest (sleep).
The amount of blood that the heart pumps in one minute is called the cardiac output. It is calculated by the amount of blood ejected with each heart beat (stroke volume) multiplied by the number of heart beats in a minute. If the heart rate is too slow to meet the physiologic requirements of the body, the cardiac output will not be sufficient to meet the metabolic demands of the body. One of two major symptoms may result. If the heart effectively stops with no heart beat, there will be no blood flow and if this is sustained for a critical period of time (10 to 30 seconds), the individual will faint. If there is a heart beat but it is too slow, the patient will be tired and weak (termed low cardiac output).
Too slow a heart beat is termed a bradycardia. Any heart rate below a rate of 60 bpm is considered a bradycardia. However, a bradycardia only needs to be treated if it is causing the individual to .have symptoms. If it is a persistent abnormality and is causing symptoms, implantation of a permanent cardiac pacemaker is often prescribed.
A pacemaker may also be referred to as a pacing system. The pacing system is comprised of two major components. One component is the pulse generator which includes the electronic circuitry and the power cell or battery. The other is the lead or leads which connect the pacemaker to the heart.
Pacemakers are described as either single chamber or dual chamber systems. A single chamber system stimulates and senses the same chamber of the heart (atria or ventricle). A dual chamber system stimulates and/or senses in both chambers of the heart (atria and ventricle).
The pacemaker delivers an electrical stimulus to the heart to cause the heart to contract when the patient's own intrinsic rhythm fails. In this way, the pacemaker can help to stabilize the electrical rhythm of the heart.
The basic function of a pacemaker can be generically described by a five letter code. The first three letters refer specifically to electrical stimulation for the treatment of bradycardias. The fifth position refers to electrical stimulation therapy for the primary treatment of fast heart rhythms or tachyarrhythmias or tachycardias. The fourth position reflects the degree of programmability and rate modulation.
The first position of the code identifies the chamber to which the electrical stimulus is delivered. If the device is not capable of bradycardia support pacing, a "O" would occupy this first position. If the unit paces in the ventricle, this is identified by a "V" while an "A" indicates that it can deliver the stimulus to the atrium. If stimuli can be delivered to either the atrium or ventricle, the letter "D" is used to reflect dual chamber stimulation.
The second position of the code identifies the chamber or chambers in which sensing occurs. Sensing is the ability of the pacemaker to recognize the intrinsic electrical activity of the heart. The letters used in this position are identical to those used in the first position.
The third position identifies the way the pacemaker responds to a sensed signal. An "I" means that the pacemaker will be inhibited. That is, when the pacemaker senses or sees an intrinsic electrical signal, it inhibits its own output pulse and resets one or more internal timers within the pacemaker's circuitry. The other basic response is represented by a "T" which means triggered. The triggered mode of response indicates that when the pacemaker senses an intrinsic electrical signal, it not only resets various internal timers within the pacemaker, it also initiates or releases a stimulus in response to that sensed event. An output pulse is said to be triggered. "D" in this position refers to both modes of sensing response. Most commonly, a sensed signal arising from the atrium and sensed on the atrial channel of a dual chamber pacemaker will inhibit the atrial output but trigger a ventricular output after a brief delay (the AV interval). If a native ventricular depolarization does not occur before the AV delay timer completes, a ventricular stimulus will be released at the end of this AV delay. If a native ventricular signal is sensed within the AV interval, the ventricular output will be inhibited and other timers will be reset. If a native ventricular signal is sensed before the atrial stimulus is released, both the atrial and ventricular output pulses will be inhibited and the various timers will be reset.
The fourth position is unique. It reflects the degree of programmability and rate modulation. It also reflects a hierarchy of capabilities. An "O" in the fourth position indicates that the pacemaker cannot be noninvasively adjusted or programmed. Programming is the ability to adjust or change the parameters of the pacemaker from outside the body without requiring a repeat operation. It is usually accomplished by a series of critically timed magnetic or radio frequency (rf) pulses controlled by a special device termed a programmer. The letter "p" in the fourth position refers to simple programmability, namely only one or two parameters can be programmed. The letter "M" in the fourth position refers to multiparameter programmability. This means that three or more parameters can be programmed, but this code doesn't identify which parameters are capable of being adjusted. The letter "C" in the fourth position refers to communicating or telemetry. Generally, all pacemakers identified by the letter "C" have multiparameter programmability. The ability to communicate means that the pacemaker has the capability to transmit information concerning its function and how it is programmed to an external device, such as a programmer. The letter "R" in the fourth position indicates that the pacemaker has rate-modulation capability, namely its rate can be automatically adjusted in response to the input from a special detector or sensor that recognizes a signal other than the basic cardiac depolarization which is processed by the sensing circuit. All pacemakers with rate modulation capability have multiparameter programmability and communication ability.
The fifth position of the code refers to special and automatic antitachycardia functions. Again, a "O" in this position indicates that it does not have this capability. A "P" refers to the ability of the device to release one or more impulses in response to a fast heart rate, or tachycardia. This is termed antitachycardia pacing and uses energy levels in the range normally used by a pacemaker, i.e., microjoules. If an "S" is used in the fifth position, it indicates that the device can deliver a shock in an attempt to terminate or end a tachycardia. A shock is a large energy pulse delivering energy 1,000,000 times that of a standard pacemaker pulse. The unit of energy for a shock pulse is joules. A "D" in the fifth position means that the device is capable of dual modes of antitachycardia response.
Returning to the basic concept of a pacemaker for treating bradycardias, most current pacing devices are called demand units. This means that they are capable of sensing the electrical activity of the heart chamber by way of the pacing lead placed in or on that chamber. The electrical signal sensed inside or on the heart is called an electrogram (EGM). The EGM is a very rapid, relatively large signal. The most rapid portion of this signal is called the intrinsic deflection (ID). Although medical personnel commonly talk about pacemakers sensing P-waves or R-waves, this is not technically correct. The P-wave and R-wave are those portions of the surface ECG corresponding to atrial and ventricular depolarization, respectively. The pacemaker sensing circuits sense the atrial or ventricular intrinsic deflection portion of the atrial or ventricular EGM from within the heart. The atrial EGM coincides with the P-wave of the surface ECG, while the ventricular EGM coincides with the R-wave of the surface ECG. For purposes of this application, the terms "P-wave" and "R-wave" will be used synonymously with the atrial and ventricular electrograms.
One of the parameters of the pacemaker that can commonly be programmed or set by the physician is the base rate. This is the lowest rate that can occur in a patient before the pacemaker will release an output pulse to initiate a cardiac depolarization followed by a contraction. If the patient's intrinsic heart rate is faster than the base rate of the pacemaker, the pacemaker will recognize the native electrical depolarization and be either inhibited or triggered depending upon how it is set and reset in its various timing cycles in response to this sensed event. If the patient's own heart beat attempts to slow below the programmed base rate of the pacemaker, the pacemaker's timing circuits (or "timers") will cause the pacemaker to release an electrical impulse at the programmed base rate, thus preventing the patient's heart rate from falling below the programmed base rate.
The interval between consecutive pacing impulses within the same chamber is termed the automatic interval or basic pacing interval. The interval between a sensed event and the ensuing paced event is called an escape interval. In single chamber pacing systems, the automatic and escape intervals are commonly identical. In dual chamber pacing systems, the basic pacing interval is divided into two sub-intervals. The interval from a sensed R-wave or ventricular paced event to the atrial paced event is called an atrial escape interval. The interval from the sensed P-wave or atrial paced event to the ventricular paced event is called the AV interval.
In the majority of individuals, the most effective heart beat is caused by the patient's own intrinsic electrical activity. A pacemaker is intended to fill in when the patient's intrinsic rhythm fails. The first pacing mode that was developed was single chamber ventricular stimulation. It was soon recognized that this resulted in the loss of appropriate synchronization between the atria and ventricles, resulting in the efficiency of the heart being compromised and the cardiac output falling, despite maintaining an adequate rate.
In those patient's whose need for a pacemaker was intermittent, i.e., where the patients had a normal rhythm between the times when pacing support was required, pacemakers were developed which were initially set to a slow rate, which slow rate could could be subsequently programmed, as required. This allowed the patient's intrinsic rhythm to slow to this very low escape rate before the pacemaker would be activated. While the patient would be protected from asystole (a total absence of any heart beat), the loss of appropriate AV synchrony combined with the slow rate was often hemodynamically compromising.
Hence, an operating modality known as "hysteresis" was developed. With hysteresis, the escape rate of the pacemaker was slower than the automatic rate. This allowed the patient's normal rhythm to persist until the rate fell below the hysteresis escape rate. When this happened, there would be one cycle of pacing at the hysteresis escape rate followed by pacing at a more rapid rate until a native R-wave occurred and was sensed to again inhibit the pacemaker.
A number of problems were recognized with hysteresis. One was confusion on the part of the medical personnel caring for the patient because the patient's intrinsic rhythm was often running at a slower rate than the automatic rate of the pacemaker. The second was that a slow heart rate often caused premature ventricular contractions (PVCs) to occur. A PVC is essentially an R-wave that occurs out of sequence, i.e., consecutive R-waves without an intervening atrial depolarization. Because the PVC would be a sensed R-wave, its occurrence would reset the pacing system to the hysteresis escape rate following each PVC that occurred, effectively maintaining a slow rate.
Based upon these two drawbacks (confusion on the part of the medical community and the repeated resetting of the pacemaker by PVCs), hysteresis was not well accepted by the medical community until such time as it was introduced as a programmable parameter capable of being enabled or disabled, and if enabled, with the degree of hysteresis being adjustable.
Since the goal of hysteresis was to allow the patient to remain in a normal rhythm with appropriate AV synchrony as much time as possible while providing pacing support at an appropriate rate only at those times when the patient required this support, hysteresis was not incorporated in the first generation of dual chamber pacing systems because such systems were designed to always provide appropriate AV synchrony. However, some physicians recognized that some patients whose heart rate would precipitously and abruptly slow not only needed a more rapid rate at these times, they also required AV synchrony. These episodes were infrequent. If one were to program the base rate of the pacemaker to the rate which was required when pacing was needed, the pacemaker would be frequently controlling the patient's rhythm even when pacing was not needed. To address this concern, some of the first generation dual chamber pacemakers were programmed to provide hysteresis in the DDI mode. This allowed the pacemaker to remain inhibited during a normal rate, to turn on only when the pacemaker was needed as represented by a precipitous and dramatic slowing of the native rhythm but once activated, to then pace in both atrium and ventricle at a more rapid rate until such time as a native R-wave was sensed to return the pacemaker to the inhibited state requiring completion of another hysteresis escape interval before it would again release atrial and ventricular output pulses.
A couple of problems were recognized in this first application of hysteresis in the DDI mode. The first was that PVCs, a limitation first noted with single chamber hysteresis systems, proved equally limiting in the dual chamber pacing mode. The second was that while pacing was required, the patient's often needed a relatively short AV delay for optimum hemodynamic function. The short AV delay could result in the pacemaker usurping control of the normal conduction system resulting in sustained periods of pacing when it was no longer required. Thus, if a long AV delay were programmed which would result in appropriate pacing system inhibition when pacing therapy was not required, it also might allow intact AV conduction when pacing was required. Such AV conduction, which is manifest by the occurrence of an R-wave, could thus reinitiate the hysteresis escape interval, causing sustained pacing at the relatively slow hysteresis escape rate. While the relatively slow hysteresis escape rate might be appropriate for one cycle, it is certainly not appropriate for sustained periods of time when pacing therapy is required.
The present invention is intended to incorporate hysteresis in the dual chamber pacing mode, specifically the DDI mode, with unique designs which: (1) protect the pacemaker timers from being reset by a PVC during sustained periods of AV pacing; (2) allow for a long AV interval when the pacing system is inhibited; (3) provide for a shorter AV .interval during AV pacing, but periodically screen or search for resumption of AV conduction during periods of pacing to determine if the pacemaker should again be inhibited.